Field of the Invention
Description of Related Art
In an endoscope system, a still image for detailed observation may be stored during observation of a moving image. The storage of a still image is performed in response to a freeze instruction from an operator. The stored still image is used for a diagnosis of a lesion. In order to facilitate the diagnosis easily, the still image is required to be a high-definition image. In order to improve workability of an operator, a display delay is required to be small.
In a wireless endoscope that transmits a still image in a wireless manner, a transmission channel band is limited. Accordingly, it is difficult to transmit a high-definition image while reducing a display delay. The wireless endoscope is driven with a battery in order to improve portability. Accordingly, a reduction in circuit scale and a simplification of processes are required for power saving.
In order to solve these problems, a wireless endoscope may separately transmit a low-resolution moving image for normal observation and a high-resolution still image for detailed observation. Patent Literature 1 discloses the following image-processing device. The image-processing device selects encoding to be performed on image data among encoding for a moving image and encoding for a still image on the basis of an instruction from an instruction unit. The encoded image data is transmitted to an external device. Patent Literature 1 also discloses an example in which transmission of a moving image can be interrupted and a high-definition still image can be instead transmitted in response to an instruction from the instruction unit during transmission of a low-quality moving image.
Aside from the above-mentioned related art, a so-called progressive encoding method which is a part of the Joint Photographic Experts Group (JPEG) standard is disclosed. In this method, a still image of a frame is encoded (corresponding to compression) and the encoded still image of the frame is divided for each frequency component. The frequency components are sequentially transmitted from a low frequency component to a high frequency component. Accordingly, the image quality of a still image to be displayed gradually varies from low image quality to high image quality. Details of the progressive encoding are disclosed in Hiroshi YASUDA, “International Standard of Multimedia Encoding,” Maruzen Co., Ltd., July, 1991, p. 24-26.